1. Field of the Invention
This invention relates to hoist mechanisms.
More particularly, the present invention relates to hydraulic hoist mechanisms for tilting dump bodies.
In a further and more specific aspect, the present invention concerns a hoist mechanism having a moving instant center of body rotation for increased efficiency.
2. The Prior Art
Hydraulic hoist mechanisms for lifting dump bodies are widely used throughout industry and come in a wide variety of types. Typically, hydraulic hoist mechanisms are used on vehicles such as dump trucks or the like, having a dump body pivotally coupled to a vehicle chassis. The dump body is moved between a lowered and a raised position by a hoist mechanism employing one or more hydraulic cylinders. Generally, a rearward end of the dump body is pivotally coupled by a rearward hinge to the vehicle chassis while the forward end is raised and lowered by the hoist mechanism. In this manner, loads of bulk material are emptied from the dump body to the rear of the dump vehicle by gravity with a minimum of effort and manpower.
In the lowered position, a dump body containing a load will be horizontally positioned relative to the vehicle chassis. To dump the load, the hoist mechanism lifts the forward end of the dump body until a desired angle is achieved. Generally a 45-50 degree angle is sufficient for delivery of material from the dump body by gravity. The influence needed to lift the dump body is referred to as torque or moment about the rearward hinge. Load torque is the torque of the weight of the dump body and load about the rearward hinge or pivot point and is greatest when the dump body is initially lifted from the vehicle chassis. Hoist torque is the torque of the hoist force about the rearward hinge or pivot point of the dump body. In order to raise the load, the actual required hoist torque is at least equal to and opposite the load torque. As will be understood by those skilled in the art, the greatest hoist torque is required when the dump body is initially lifted from the vehicle chassis. This is generally referred to as the "breakaway point". As the dump body pivots upward, the hoist torque required reduces. The required hoist torque can be seen as the series of lines in the graph illustrated in FIG. 5 generated by the well known NTEA (National Truck Equipment Association of Detroit, Mich.). The axis of the graph are hoist torque and body angle in degrees. Viewing the graph, it is easily seen that the hoist torque required at the breakaway point is the greatest. With this in mind, the most common types of hoist apparatus may be addressed.
A simple, straight forward type of hoist mechanism is the direct push hoist. The simplest of these is one or more hydraulic cylinders coupled between the vehicle chassis and the dump body. To lift the dump body, the hydraulic cylinder pushes directly against the dump body. For this type of hoist, determining the hoist torque is straight forward. There are two primary factors involved when determining the hoist torque available. These factors are the cylinder force and the length of the lever arm to which the force is applied. The product of this force and distance gives the available hoist torque. Cylinder force can be shown by an arrowed line extending in the direction of piston movement. The length of the lever arm is the length of a line drawn perpendicularly to and extending from the line of cylinder force to the point around which the dump body pivots (rear hinge). Given a desired maximum hoist torque, it will be understood that as the length of the lever arm increases, the cylinder force can be reduced. This translates into reduced cost by being able to use smaller diameter less expensive cylinders. Conversely, if the lever arm shortens, a larger cylinder force and therefore a larger cylinder diameter is required, assuming a constant operating pressure, to provide the same hoist torque. It will be appreciated by one skilled in the art that as the hydraulic cylinder is moved away from the rear hinge towards the front of the vehicle chassis, the lever arm increases.
At first it would appear that a relatively small cylinder could be placed proximate the front of the dump body and substantially upright with respect to the vehicle chassis. This would provide a long lever arm thereby reducing the cylinder force needed. Other considerations, however make this impractical. To begin with, a hydraulic cylinder has a limited stroke length making it impossible to raise the dump body to the desired dump angle, and furthermore the cylinder would take up a great deal of space vertically, prohibiting its use as an underbody hoist.
The problem of stroke length has been answered to some degree by the use of telescoping cylinders. These cylinders extend a great deal further, allowing them to be positioned further forward and still lift the dump body to a sufficient dump angle. The drawback of using these cylinders is their substantially higher cost, and their complexity which reduces reliability and increases maintenance and down time. Furthermore, they still take up a large amount of vertical space prohibiting underbody mounting. Underbody mounting, refers to mounting the hoist mechanism between the floor of the dump body and the frame of the vehicle.
A hydraulic cylinder could be placed closer to the rear hinge where its stroke length would be sufficient to lift the dump body to the desired dump angle. This however would require more cylinder force and therefore a larger more expensive cylinder and structure. Again, the vertical height is large, prohibiting underbody mounting. The vertical height required could be reduced if the hydraulic cylinder is angled toward the rear hinge, however the lever arm is again reduced requiring more cylinder force.
Furthermore, direct lift cylinders are extremely inefficient. The most efficient hoist mechanisms would utilize the maximum force of the hydraulic cylinder throughout the stroke length thereof. As shown previously the greatest hoist torque is required at lift off and reduces as the dump angle increases. Therefore, in a direct lift mechanism, the cylinder force must have a maximum force great enough to satisfy the hoist torque requirement at lift off. After lift off, the hoist torque required drops off, but the available hoist torque provided by the hoist mechanism remains substantially constant. The large cylinder force needed at lift off to provide sufficient hoist torque is no longer needed. This is an inefficient hoist mechanism.
To provide a hoist mechanism which is mounted under a dump body (underbody hoist), and overcome the dilemma of balancing the cylinder force with the length of the lever arm as well as limit mounting height, various types of hoist mechanisms have been developed. The most common and effective types include scissor link hoist mechanisms and lost motion hoist mechanisms. Generally, the basic problem with underbody hoist mechanisms is that they have the least amount of vertical space when the most hoist torque is required and the most vertical space when the least hoist torque is required.
The scissor type hoists include a link pivotally coupled to the dump body, a link pivotally coupled to the vehicle chassis and the other link, and a hydraulic cylinder which actuates the linkage. Basically, the scissor linkage permits the hydraulic cylinder to operate in a more horizontal orientation reducing mounting height and allowing the hoist to be mounted under the dump body. The linkage transmits the cylinder force generally to a point on and at an angle with respect to the dump body such that the lever arm is substantially longer than the positioning and angle of the cylinder in a direct lift hoist would normally allow. In this manner a vertically compact hoist mechanism is achieved.
The drawback to scissor hoists is that they are very inefficient, with the hydraulic cylinder being unnecessarily large throughout most of the lift with the maximum cylinder force only required at lift off.
Lost motion hoist mechanisms are generally similar to scissor mechanisms, but employ some sort of lost motion during the initial lift off, thereby reducing the cylinder force required at lift off while maintaining the needed hoist torque. Lost motion hoist mechanisms employ a variety of different elements, such as slides, rollers or cams to produce the lost motion. These hoist mechanisms employ the lost motion through lift off, then revert to a normal scissor type lift mode. The benefit of these hoist mechanisms is a reduced cylinder force requirement at lift off. The reduced force increases efficiency since a smaller cylinder can be used with the available hoist torque more closely matching the load torque.
While the available torque curves of this type generally follow the load torque curve closely near the lift off, they begin to diverge when the lift shifts to standard scissor type. Again this divergence of the torque curves relates to inefficiency. Furthermore, the lost motion type uses slides, rollers and cams which can cause problems, reducing reliability and increasing maintenance and down time.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide a new and improved hoist mechanism.
Another object of the present invention is to provide an efficient hoist mechanism.
And another object of the present invention is to provide a versatile hoist mechanism having a low mounting height, thereby lowering the height of the dump body and reducing the work needed to load same and also reducing the shipping height of the body and hoist.
Still another object of the present invention is to provide a hoist mechanism which, due to high operative efficiency, utilizes smaller, less costly components and is therefore relatively cost effective.
Yet another object of the present invention is to provide a hoist mechanism which can be mounted on conventional truck chassis.
Yet still another object of the present invention is to provide a hoist mechanism simple to mount on a truck chassis.
A further object of the present invention is to provide a stable hoist mechanism.
And a further object of the present invention is to provide an easily maintainable hoist mechanism.
Yet a further object of the present invention is to provide a hoist mechanism generating a torque which closely matches the load torque throughout the entire dumping cycle.
And another object of the present invention is to provide a versatile hoist mechanism where a range of loads may be carried and dumped by changing the cylinder size, while using the same mass produced linkage and support components.
A further object of the present invention is to provide a hoist mechanism which is light weight, allowing an increase in available payload capacity.